How important is molecular rigidity for the complex stability of artificial host-guest systems? A theoretical study...

Arginine forms much less stable dimers than 2-(guanidiniocarbonyl)-1H-pyrrole-5-carboxylate although the principal binding interactions are very similar. The reasons for this difference are addressed in this work by state-of-the-art ab initio computations. The investigation shows that the extraordinary high stability of the 2-(guanidiniocarbonyl)-1H-pyrrole-5-carboxylate dimer results to about 50 % from the rigidity of its monomer. Within this study monomer and dimer conformers of arginine were calculated leading to new low lying structures which have not been reported before as well as new global minima are predicted. In these structures stacking interactions with the guanidinium moiety are especially important. For the monomer we predict the energy minimum to be the canonical form with the lowest lying zwitterionic structure being only 9 kJ mol(-1) less stable. During the course of these calculations we found that DFT did not predict the structures and their relative energy correctly in comparison to perturbation theory (MP2) and some potential reasons for the failure of DFT in these cases are discussed. Vibrational frequencies of the various structures are presented and a suitable wavenumber region for an experimental determination of the global minimum of the arginine monomer is identified. The effect of molecular rigidity on the self-assembly is probed using a local minimum of the arginine monomer which does not possess any intramolecular stabilizing effects. Our results suggest that the deliberate control of the conformational flexibility is a powerful instrument to steer the complex affinity of artificial hosts.